Gas-separation modules are commonly used to selectively extract a particular gas from a gas mixture. Two of the most common gas-separation modules are polymer membranes and metallic composites. Polymer membranes provide an effective and cost-efficient option for separating a gas at low temperatures. Where separations must be performed in conjunction with high-temperature processing, however, polymer membranes are generally unsuitable because they tend to thermally decompose.
The development of high-temperature processing along with tighter environmental regulations requires utilization of gas-separation modules that provide high fluxes, high selectivity of separation and the ability to operate at elevated temperatures. Instead of polymers, metallic composite modules are widely employed to serve these needs. A composite module consists of a metallic membrane having selective gas-permeability mounted on a porous metallic substrate for support. Alternatively, the module can be a tube formed purely of palladium.
An area of high-temperature gas separation that is of particular interest is the separation and purification of hydrogen gas from a reaction gas mixture. A composite module for selectively separating hydrogen gas at high temperatures includes a palladium (Pd) membrane mounted on a porous metallic substrate. The palladium membrane is permeable to hydrogen but not to other gases. When hydrogen gas (H.sub.2) contacts the membrane, the hydrogen molecules dissociate and hydrogen atoms diffuse into the membrane. Accordingly, hydrogen can selectively pass from a surrounding atmosphere through the palladium membrane to the porous substrate. The selectively-extracted hydrogen atoms then reform into H.sub.2 gas and pass through the pores of the porous substrate and into a volume on the opposite side of the module.
Nevertheless, the effective life of a typical module having a palladium membrane bonded to a porous metallic substrate often is limited by diffusion of the substrate into the membrane which decreases the permeability of the membrane to hydrogen. The rate of diffusion of the substrate is greatest when the substrate is at or above its "Tamman" temperature. A metal lattice at its Tamman temperature is subjected to considerable thermal (atomic) vibration. If there is an interface between two metals, such thermal vibration significantly increases the mobility of metal atoms and their consequent diffusion. The Tamman temperature of a material is equal to one-half of its melting temperature (in K). Palladium and stainless steel have melting points of 1552.degree. C. (1825 K) and 1375-1400.degree. C. (1648-1673 K), respectively. The corresponding Tamman temperatures are about 640.degree. C. (913 K) and 550-560.degree. C. (823-833 K), respectively. The lower of these temperatures determines the temperature where a significant increase in intermetallic diffusion occurs. Accordingly, at temperatures around 550.degree. C., considerable thermal vibration and diffusion of stainless steel components into the palladium is expected. The alloy created by the diffusion of stainless steel components into the palladium will have reduced hydrogen permeability.
One solution to this problem has been to use a ceramic substrate which will exhibit less diffusion than a purely metallic substrate. Ceramic substrates, however, are typically more brittle than metallic substrates. Further, ceramic substrates are more difficult to fabricate and are also more difficult to join to other components in a gas-separation system.
Gas-separation modules formed purely of palladium have also been used. The elimination of the metallic substrate removes the problem of intermetallic diffusion. However, a monolithic palladium module is very expensive to produce. It must also have a much greater thickness than a composite module to provide the mechanical strength that is desired. This increase in thickness reduces the flux of hydrogen that can be established through the module.
Another approach is to deposit a thermally-stable material on the metallic substrate before applying the selectively-permeable membrane. In U.S. Pat. No. 5,498,278, issued to Edlund, an embodiment is disclosed wherein the thermally-stable material is a woven or non-woven fabric laminated onto the metallic substrate. In another embodiment, disclosed in Gryaznov, et al., Preparation and Catalysis over Palladium Composite Membranes, 96 APPL. CATAL. A: GENERAL 15 (1993), an intermediate layer is provided by depositing zirconia, magnesia, tantalum oxide, or tungsten onto the substrate by a magnetron sputtering process. These approaches, however, are complex. Further, the intermediate layer often lacks uniformity, thereby causing the module to be vulnerable to diffusion through gaps in the intermediate layer.